Class UserGroupInformation

User and group information for Hadoop.
This class wraps around a JAAS Subject and provides methods to determine the
user's username and groups. It supports both the Windows, Unix and Kerberos
login modules.

reloginFromKeytab

Re-Login a user in from a keytab file. Loads a user identity from a keytab
file and logs them in. They become the currently logged-in user. This
method assumes that loginUserFromKeytab(String, String) had
happened already.
The Subject field of this UserGroupInformation object is updated to have
the new credentials.